In order to defrost electric windshields in motor vehicles quickly, proposals have already been made to feed electric windshield with electricity at a raised-voltage in order to ensure effective defrosting action in a short length of time, not exceeding a few minutes.
Such high-speed defrosting systems are described, for example, in French patent applications Nos. 86 07403 and 87 04504 in the name of the present applicant.
In the above-mentioned systems, a raised feed voltage (with raised-voltage conditions reaching as much as 5 to 6 times the nominal charged voltage of the motor vehicle battery) is applied to the electric windshield by changing the regulation level for the rectified output voltage from the alternator.
In the above-mentioned systems, the change in regulation level is controlled in a manner which does not take account of the state of the switching device which directs electrical energy from the alternator to the battery for the purpose of recharging the battery or to the electric windshield for the purpose of defrosting it.
In particular, if there is a switching fault, the abovementioned systems are capable of overcharging the battery which can have damaging or dangerous consequences, for example destroying the vehicle lamps, running the risk of fire, or even the risk of the battery exploding.
Such a situation may arise when the above-mentioned defrosting system is put into action while a fault in the switching device maintains an unwanted connection between the rectified output from the alternator and the battery.
In addition, in the above-mentioned systems, the function of reactivating the regulator remains active so long as the defrosting system and thus the switching device are in action, i.e. for several minutes.
Consequently, if there exists a fault short-circuiting the feed line to the electric windshield, the alternator delivers all of its energy to the short circuit throughout the defrosting period. This situation also runs the risk of damaging the vehicle, in particular by fire.
The object of the present invention is to implement a system for controlling raised-voltage defrosting of the electric windshield in a motor vehicle, in which the abovementioned drawbacks are eliminated.
Another object of the invention is in particular to implement a control system for raised-voltage defrosting of the electric windshield of a motor vehicle which eliminates the danger of the regulator operating to provide raised-voltage regulation of the rectified output from the alternator while the battery is connected to the switching device at the rectified output from the alternator.
Another object of the present invention is to implement a control system for raised-voltage defrosting of the electric windshield in a motor vehicle in which the operation of the regulator to provide raised-voltage regulation of the rectified output from the alternator is not, in fact, enabled unless a connection is established between the alternator and the feed line to the windshield.
Another object of the present invention is to implement a system for controlling raised-voltage defrosting of the electric windshield in a motor vehicle in which the time actually required for reactivating the alternator, during which time the inductor of the alternator is excited in full field, is limited to a length of time equal to a fraction of the defrosting time.